Retroreflectors are known devices which are often used as transponders. Retroreflectors receive light and reflect the light back in the direction from whence it came. A passive retroreflector returns light with the same general characteristics of the incident light, preferably with relatively high directional gain and relatively little spreading of the incident light beam. In contrast, an active retroreflector may include an emitting device that can generate a user definable signal in response to the incident radiation beam.
Retroreflectors have found a wide variety of applications in numerous fields including communications systems, monitoring systems, and in-flight refueling systems. Examples of different types of retroreflector structures include a corner-cube reflector, a horn reflector, a parabolic dish reflector, and a parabolic cylinder reflector.
Another application of retroreflectors is in the task of identifying friend-or-foe (IFF) in a battleground setting. Since the evolution of weaponry which allowed opposing forces to fight through the exchange of the instrumentalities of war at a distance, fratricide killing has been a problem. IFF tasks are a delicate comprise between secure, ambiguous identification and the maintenance of stealth positions. In typical IFF systems, a radio or microwave frequency request is made by an interrogation unit such as a plane or tank and a corresponding signal is returned by the targeted unit. This is normally achieved by a transponder on the targeted unit that emits a coded return signal when the interrogation request is received. Other systems merely re-radiate or reflect the incident interrogation request, while some systems modulate the re-radiated or reflected signal in an distinctive manner. The interrogation unit then deciphers the received signal to determine if the targeted unit is a friend or foe. However, by emitting (i.e., reflecting) a broadly directed response that is designed to have a sufficient strength to reach the interrogation unit, some of the radiation may be detected by other units of the opposing force which may reveal the position of the targeted unit.